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Calculate the pH for
(a) 0.02 M NaOH
(b) 0.10 M NH4OH
Solution:
pH means "power of hydrogen". Some chemists say that pH means "potential of hydrogen". It is a measurement of acidity or alkalinity of a solution and it has a scale from 0 to 14. If pH is ranging from 0 to 6, then the solution is acidic. If pH is 7, then the solution is neutral like water, for example. If pH is ranging from 8 to 14, then the solution is alkaline. The pH of any acidic solution can be calculated by using the equation,
where [H+] is the concentration of hydrogen ion in molarity.
The pOH of any basic solution is
where [OH-] is the concentration of hydroxide ion in molarity.
Hence, the pH of any basic solution can be calculated by using the equation,
(a) For 0.020 M NaOH solution
Since NaOH is a strong base, then it is completely ionized into sodium ion and hydroxide ion as follows
In a liter of solution, 0.02 moles of NaOH is ionized completely into 0.02 moles of Na+ and 0.02 moles of OH-. Hence, the concentration of [OH-] is 0.02.
The pOH of 0.02 M NaOH is
Therefore, the pH of 0.02 M NaOH is
If the pH of a base is closer to 14, then it is a strong base.
(b) For 0.10 M NH4OH solution
Since NH4OH is a weak base, then it is partially ionized into ammonium ion and hydroxide ion as follows
In a liter of solution at equilibrium,
where x is moles of NH4OH that partially ionized. The ionization constant for NH4OH is given by the equation
From Table of Ionization Constants of Acids and Bases, Kb = 1.8 x 10-5 for ammonium hydroxide. The amount of NH4OH that partially ionized is
Since the value of Kb is less than 1 x 10-3, then we can neglect x at the denominator as follows
Hence, the concentration of hydroxide ion is
The pOH of 0.01 M NH4OH is
Therefore, the pH of 0.01 M NH4OH is
If the pH of a base is closer to 7, then it is a weak base.